The AIDS epidemic has generated substantial, largely unanticipated demand for hospital care, with hospitalizations measured in the hundreds of thousands and bed days in the millions. A number of reports suggest that AIDS-related hospital utilization has been highly concentrated, not only geographically but in a small percentage of hospitals within geographic areas. Concentration of care may be beneficial if it leads to improved outcomes or harmful if access to care is limited. Concentration may cause serious financial problems for hospitals because of under-reimbursement for AIDS-related care. This study seeks to measure concentration within 27 specific MSA's accounting for 60% of U.S. AIDS cases using hospital discharge data. Measures of inequality of a distribution including the Gini index will be employed to study trends in concentration (1983-1989). The relationship of concentration to patient and hospital characteristics will also be assessed.